Spiral antenna with arms of different lengths for polarization change



June 9, 1964 J. A. KAISER, JR., ETAL SPIRAL ANTENNA WITH ARMS OF DIFFERENT LENGTHS FOR POLARIZATION CHANGE Filed April 5, 1962 71/1 /1101 17/11 A ll/11111 FIG.3

IN VEN T 0E5,

JUL/U5 A. Aw/sge, J/a

JOHN A! 1/7/25 United States Patent Ofifice 3,137,002 Patented June 9, 1964 3,137 ,002 SPKRAL ANTENNA WlTH ARMS OF DEFERENT LENGTHS FOR PGLAREZATION CHANGE Julius A. Kaiser, In, Kensington, and John H. Little, Silver Spring, Md, assignors to the United States of America as represented by the Secretary of the Army Filed Apr. 5, 1962, Ser. No. 185,490 6 Claims. (Cl. 3438S4) (Granted under Title 35, U.S. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment to us of any royalty thereon.

This invention relates to the field of antennas, and more particularly to the field of spiral antennas.

In many applications of radio frequency energy devices, it is desirable to have an antenna which is capable of generating a circularly polarized beam; or a linearly polarized beam having a variable polarization orientation. Such results have previously been obtained typically by the use of two dipole antennas placed at right angles to each other.

The spiral antenna, comprised of two filaments of equal length, has been shown to be capable of producing a broad, circularly polarized beam centered on the spiral axis. This operation is described in pending application Serial No. 163,369, filed December 29, 1961, by Julius A. Kaiser, Jr. The signal generated by such an antenna will normally exhibit a polarization sense (direction of electric vector rotation) which corresponds to the winding sense of the spiral.

Applicant has discovered that such aspiral antenna can be modified so as to be capable of producing either sense of circular polarization, either alternately or simultaneous- 1y.

Accordingly, an object of this invention is toprovide a circuit for energizing a spiral antenna so as to cause it to generate either sense of circular polarization.

A further object of this invention is to energize a spiral antenna so as to cause it to produce simultaneously both senses of circular polarization.

It is still another object of this invention to provide novel means for measuring spiral antenna eificiency.

Another object of this invention is to provide means for improving the axial ratio of the radiated field from a spiral antenna.

These and other objects, features, and advantages of this invention will be apparent from the following specification taken in conjunction with the drawings, in which:

FIG. 1 is a diagrammatic representation of an Archi medean spiral antenna modified in accordance with the instant invention;

FIG. 2 is an exploded view of a system for practicing the instant invention, and

FIG. 3 is a. side view of the system of FIG. 2.

The two-wire spiral antenna behaves generally as though it were a two-wire transmission line which gradually, by virtue of its spiral geometry, transforms itself into a radiating structure or antenna. It is will known that a two-wire transmission line of narrow spacing, relative to wave length, and of any length yields a negligible amount of radiation when excited at its terminals. This is due to the fact that currents in the two wires of the lines at any normal cross-section are always 180 out of phase so that lines of force emanating from the electric charges on one line are terminated on the equal but opposite polarity charges of the other line, so that radiation is effectively prevented.

Suppose now that a two-wire transmission line is formed into the spiral configuration of FIG. 1. Let P be a point on one wire of the transmission line at a-distance measured along the wire from the input terminal A. Then the point Q on the other wire at the same distance from the input terminal B is situated diametrically opposite the point P in respect to the center 0, and both P and Q lie on the same circle centered at O. This implies that the point P and its neighboring point P on the other wire directly alongside P lie at such distances from A and B, respectively, that the difference of these distances is precisely the arc length QP' along the spiral. If Ar, the spacing between wires is much smaller than r, the arc length QP' is approximately equal to M. This difference in wire length is constant regardless of the number of turns within r, provided only that the spacing between wires is uniform.

The configuration of the spiral antenna can take several forms, some of which are the Archimedean spiral, the logarithmic spiral and a spiral made up of a series of linear segments, known as the square spiral.

There are two items of interest that attach to a particular point on the two-wire spiral line: the total difference in wire lengths of the two-wire line to the point, and the circumference or path length of the particular turn on which the point lies. For the circular spiral where the wires are equally spaced, at a point whose radial distance from the spiral center is r, the difference in line length equals M and the circumference equals 21rr. When r is h/Zn', the phase change is A/ 2 and the circumference is A. Assuming that each wire supports a progressive wave of current and that these current waves are antiphase at the input terminals A and B, it is clear that the difference in phase of the two current elements at any point PP on the two wire line, measured in radians is (the input phase difference) Thus neighboring current elements start anti-phase at the feedpoints A and B, and gradually come into phase as they proceed outward along the spiral two-wire line. When r is )\/21r, these currents are precisely in phase and radiation is a maximum. When the two wires are relatively close together, and the spiral radius increases slowly as one moves along the spiral (e.g., if the spacing between any two wires were the small distance Ar, then segments of the same wire which are 360 apart would only have a separation of 2Ar), the currents in two wires may be considered to be in phase over an entire 360 segment of the spiral at the one wave length circumference. This behavior of the spiral antenna is fully described in Patent No. 2,947,000, issued to Arthur E. Marston and Julius A. Kaiser, Jr., on July 26, 1960. Since the wave producing the radiation in the above described antenna travels from the center terminal outward to the ends of the antenna wires, the sense of polarization of the wave produced by this antenna will be in the direction of the winding sense of the spiral antenna.

When the spiral antenna is excited in an anti-phase manner and when radiation at the one wavelength circumference is not suppressed, most of the energy conducted along the spiral will be radiated at the one wavelength circumference and very little energy will travel beyond this region. Therefore, the spiral antenna will behave in the above noted manner as long as its diameter is greater than 7\/1r.

Previous spiral antennas were constructed so as to have two equal length filaments. The present invention involves the modification of one of these filaments so as to make it )\/4 shorter than the other filament. If the total diameter of an antenna modified in this manner is made less than 2 \/1r, but greater than )\/1r, then the antenna, when excited at its input terminals with in-phase currents, will produce a beam having a sense of circularity and radiation would occur.

. 3 v of polarization opposite to thewinding senseof the spiral. In other words, when'such an antenna is used, the sense of circular polarization of a beam produced by anti-phase input currents will be opposite to that of a beam produced by in-phase currents. 7 v

The mechanism. of this behavior can bestbe understood by reference to 'FIG. 1, wherein the filament '10 has been A and B. Q The in-phasecurrents would thenbe conducted along conductors 18 and Hand would produce a 'signa atlterminal 3 of ring network 16. g

modified so as to be M4 shorter than the filament 11,-:-

Since the modified antenna has a diameter greater than )\/'II', this shortening of the filament will have little effect on the nature of the radiation .produ'cedby anti-phase input since most of the energy introduced into the antenna" is radiated at the one wavelength circumference.

The shortening of one filament of the spiralantenna Q I has its greatest eflect when the antenna is excited at its These currents travel along the spiral'filaments until reaching the one wavelength circumference, at which position they have. achieved an anti-phase relationship, so that only negli- J ggible radiation occurs. These currentsthen proceedalong the filaments until reaching the ends thereof at which input terminals with in-phase currents.

place they are reflected back toward the input terminals. Because one filament of the modified antenna is M4 longer than the other, the current in that filament travels M4 further than the current in the other filamentin at terminal 1 of ring network-16.

Since the amplitudes of the reflected currents'returning to input terminals A and B are usually very small, for many applications they c'lan be neglected. ;In' one'such reachingthe outer ends thereof, and another M4 further 7 than the current in the other filament after reflection,

of the antenna, and therefore opposite totne sense of polarization of the beam produced by the same antenna when its input terminals are excited with anti-phase currents. Radiation of both senses of circularpolarization occurs from-the same region ofthe spiral, which indicates beam having a sense of polarization the same the wind- .ing senseof the.anten'na. Any currents which are not that the radiation patterns should be'identical except for the sense of rotation. The frequency range of the'device' when it is receiving in-phase signals is limited by the A wavelength filament length difierence.

Referring now to the exploded view of FIG. 2 and the assembled side view of FIG. 3, the invention is shown as comprising a two-wire Archimedean spiral antenna 20- consisting of wires 10 and 11. Ring network '16, consisting of input terminals 1 and 3, output terminals'2 and 4 and conducting strip 17, is a standardv microwave component in which current entering terminal '1 arrives. at terminals 2 and 4 in phase opposition while current entering terminal 3 arrives at terminals 2 and} in-phase. Metal plate 13 is provided for use as a ground plane to insure launching of the energy onto the spiral. Terminals 2 and 4 of the ring network are connected to the'input e the ring network 16, andthe terminal 3 is open-circuited."

On the other hand, if it weredesired that'the antenna were to produce a beam having the opposite sense, of

, polarization, current would be'sentinto terminal 3 of ring network 16' and the current would consequently reach terminals A and B in phase, These currents would then I' travel outward along the spiral and reachtheone wave length circumference anti-phase,lso that no radiation would occur. The currents would continue onto the ends of their 'respectivewires and would be reflectedback t toward theinput terminals. Because of the phase shift caused by the unequal wire lengths,the reflected currents. I

would return tofthe onewavelengthcircumference 'in phase.' This would cause the antenna to, radiate a circularl'y polarized beam having a'sense of polarization 0pposedto the winding sense of the spiralantennai Any residual current remaining'after the currents traverse the one wave length circumference"would return to terminals; j A and B anti-phase. These currents would beconducted e along conductors 18 a'nd119 and would appear as a signal application equal currents may be fed simultaneouslyinto arms 1 and 3 of the ring network in order to produce simultaneously the two opposite senses of circular polarization.

mined by the relative phases of'the two input signals.

The axialratio, or circularity, 'of the radiated field can beimproved by the absorption of anyreflected currents appearing at input terminals A and B. For example, cur rents fed into arm 1 of the ring network '16result in a radiated in the first pass of the currents traveling outward are reflectedand appear at terminal 3 of the ring, Ifterminal 3 is then connected to a matched t'errnination,

all of these reflected currents will be absorbed and there will he no re-reflection of these currents. Likewise, for

inputs to terminal 3 of the ring 16, the terminal 1 could :be terminated by a matchedtermination.

This'will result in a linearly polarized output wave having a direction of polarization, which'isdet'er- V Inorder to understand why the use of a matched ter- 7 V.

mination improves the axial ratio of the radiated signal,

we will examine a signal which would be produced if the terminal were terminated in a mismatch.'. Assume first that an input signal is inserted at v:terminal 1' of A signal entering ring network 16 atterminal lca'uses equal but opposite polarity signals to appear at the *ter:

minals 2 and 4. These signals are conducted to input terminals Ajand' B'anti-phase; The currents proceed along'the spiral until they reach the one wavelength terminals A and B of the two-wire spiral by the inner.

conductors 18 and 19 of coaxial cables having outer conductors 14 and 15, respectively, which are electrically attached to the ground'plane 13. Dielectric plate 12 is provided to insulate the ground plane 13 from'the antenna when the components are assembled as'shown in 'FIG. 3.

If it were desired to operate the antennaso as to produce a sense'of polarization in the direction of the spiral winding sense, current would be fed into arm'l of the ring network 16. The current would then reach the terminals A and B anti-phase and, as described above, at the one wavelength circumference the current would be in-phase V A small residual" current would continue on to the ends of the spiral and be reflected back to the one wavelength circumference. The

unequal lengths of the two wires would cause the re circumference, at which'position they generate a circularly; polarized beam'havinga sense. of polarization'in the direction of the winding sense of the antenna. Any residual currents"continue onto the ends of the antenna wires where they are reflected back towards input terminals A and B. Because of the unequal length of the two wires, thereflected currents appear at the one wavelength circumference in anti-phase, so that, no radiation therefrom occurs. These signals continue on until they reach'input terminals -A and Him phase. currents will then be conducted along conductors 18 :and '19 to terminals 2 and 4 of the ring network 16. Since they are in-phase, these-signals can only produce.

The reflected an output at terminal '3. Since terminal 3 is terminated in a mismatch, some currents will. be re-reflected back along conductors 18 and 19 and will arrive'at terminals flected" current to be anti-phase at the one wavelength,-

circumference. These reflected-waves would then reach an in-phase position when they reach the input terminals A and Bfin'phaseI These in-phase currents willagain travel to the'ends of the wires of the spiral antenna 20,- arriving at the one wavelength circumference, antiends of wires 10 and 11 of the antenna 20 and back to ring network 16. Thus, the elliptical polarization causedv by such re-refiection would be eliminated.

The spiral antenna efficiency can be determined with the circuit of FIG. 2 by purposes surpressing radiation. Thiscan be accomplished by placing a highly conducting surface parallel and close to the spiral surface. In this manner, currents entering arm 1 appear entirely at arm 3, minus any copper and dielectric losses suffered in traveling to the end terminals of the spiral and back. The copper losses within the antenna itself are thereby separated from the radiation losses. The determination of the input mismatch by conventional methods then completely determines the spiral antenna efliciency.

' It might be noted that while the simultaneous insertion of equal amplitude inputs to terminals 1 and 3 of ring network 16 produced linear polarization, the simultaneous insertion of unequal amplitudes at these two terminals will result in an elliptically polarized signal. By thus varying the phases and amplitudes of the two simultaneously applied input signals, an elliptically polarized signal having any arbitrary elliptieity angle and orientation angle may be produced.

It will be apparent that the embodiments presented are only exemplary and that various modifications can be made in construction and arrangement within the scope of the invention as defined in the appended claims.

I claim as my invention:

1. An Archimedean spiral antenna capable of producing a circularly polarized beam of either sense for a given input signal frequency comprising:

(a) a first spiral wire filament, and

(b) a second spiral wire filament identical with said first wire filament and having the same axis as said first filament, but rotated 180 with respect to said first filament,

(c) said second wire filament diifering in length from ti said first wire filament by an amount equal to A of a wavelength of a wave having said input signal frequency. 2. An Archimedean spiral antenna as recited in claim 1 wherein the diameter of said antenna is greater than A/fl', but less than 2 \/1r, where a is the wavelength of said input signal.

3. An antenna system capable of producing an electromagnetic beam of either sense of polarization comprising: (a) a spiral antenna as recited in claim 2 said antenna further comprising a first input terminal at the innermost end of said first wire filament and a second input terminal at the innermost end of said second wire filament, and

(b) means connected to said first and second inputterminals and adapted to permit said antenna to selectively radiate a circularly polarized signal having either sense of polarization.

4. A system as recited in claim 3 wherein said means connected to said first and second input terminals is further adapted to permit two input signals to be introduced at said antenna inputs simultaneously, in such a manner as to cause said antenna to radiate a beam which simultaneously comprises two circularly polarized signals having mutually opposite senses of polarization.

5. A system for radiating a polarized electromagnetic beam comprising:

(a) a spiral antenna as recited in claim 2, said spiral antenna further comprising a first input terminal connected to the innermost end of said first wire filament and a second input terminal connected to the innermost end of said second Wire filament, and

(b) means for selectively applying currents to said input terminals which are either in phase or antiphase.

6. A system as recited in claim 5 wherein said means for applying currents to said input terminals is further adapted to apply both in phase and anti-phase currents to said input terminals simultaneously.

References Cited in the file of this patent UNITED STATES PATENTS 

1. AN ARCHIMEDEAN SPIRAL ANTENNA CAPABLE OF PRODUCING A CIRCULARLY POLARIZED BEAM OF EITHER SENSE FOR A GIVEN INPUT SIGNAL FREQUENCY COMPRISING: (A) A FIRST SPIRAL WIRE FILAMENT, AND (B) A SECOND SPIRAL WIRE FILAMENT IDENTICAL WITH SAID FIRST WIRE FILAMENT AND HAVING THE SAME AXIS AS SAID FIRST FILAMENT, BUT ROTATED 180* WITH RESPECT TO SAID FIRST FILAMENT, (C) SAID SECOND WIRE FILAMENT DIFFERING IN LENGTH FROM SAID FIRST WIRE FILAMENT BY AN AMOUNT EQUAL TO 1/4 OF A WAVELENGTH OF A WAVE HAVING SAID INPUT SIGNAL FREQUENCY. 